The principal greenhouse gases whose longwave radiative effects are
included in CAM 3.0 are HO, CO, O, CH, NO,
CFC11, and CFC12. The prediction of water vapor is described elsewhere
in this chapter, and CO is assumed to be well mixed. Monthly O fields are
specified as input, as described in chapter 7. The
radiative effects of the other four greenhouse gases (CH, NO, CFC11,
and CFC12) may be included in CAM 3.0 through specified concentration
distributions [91] or prognostic concentrations [26].

The specified distributions are globally uniform in
the troposphere. Above a latitudinally and seasonally specified
tropopause height, the distributions are zonally symmetric and
decrease upward, with a separate latitude-dependent scale height for
each gas.

Prognostic distributions are computed following Boville et al. [26].
Transport equations for the four gases are included, and losses have
been parameterized by specified zonally symmetric loss frequencies:
.
Monthly averaged loss frequencies, , are obtained from
the two-dimensional model of Garcia and Solomon [59].

We have chosen to specify globally uniform surface concentrations of
the four gases, rather than their surface fluxes. The surface
sources are imperfectly known, particularly for CH and NO in
preindustrial times. Even given constant sources and reasonable
initial conditions, obtaining equilibrium values for the loading of
these gases in the atmosphere can take many years. CAM 3.0 was designed
for tropospheric simulation with relatively coarse vertical resolution
in the upper troposphere and lower stratosphere. It is likely that the
rate of transport into the stratosphere will be misrepresented,
leading to erroneous loading and radiative forcing if surface fluxes
are specified. Specifying surface
concentrations has the advantage that we do not need to worry much
about the atmospheric lifetime. However, we cannot examine observed
features such as the interhemispheric gradient of the trace gases. For
climate change experiments, the specified surface concentrations are
varied but the stratospheric loss frequencies are not.

Oxidation of CH is an important source of water vapor in the
stratosphere, contributing about half of the ambient mixing ratio over
much of the stratosphere. Although CH is not generally oxidized
directly into water vapor, this is not a bad approximation, as shown
by Le Texier [103]. In CAM 3.0, it is assumed that the water
vapor (volume mixing ratio) source is twice the CH sink. This
approach was also taken by Mote et al. [129] for middle atmosphere
studies with an earlier version of the CCM. This part of the water
budget is of some importance in climate change studies, because the
atmospheric CH concentrations have increased rapidly with time and
this increase is projected to continue into the next century (e.g.,
Alcamo et al. [1]) The representation of stratospheric water vapor in
CAM 3.0 is necessarily crude, since there are few levels above the
tropopause. However, the model is capable of capturing the main
features of the CH and water distributions.